Early devices to vary the displacement of vane pumps involved the deliberate offset of the rotational center of the vane rotor with respect to the geometrical center of the circular outer case. The amount of offset would then control the swept volume of the pump and thereby provide a desired volumetric output for each rotation of the rotor. Several problems with this design limited its use.
First, the pressure unbalance caused by the hydraulic-based force on the radial cross-section of the rotor and vanes at the axis viewed from the radial perspective severely limited the power capability and power density of these pumps and resulted in very heavy, inefficient, and cumbersome devices. Second, the centrifugal force of each vane during high speed rotation caused severe wear of the vane outer edge and the inner surface of the outer containment housing.
Later fixed displacement designs were conceived around the concept of pressure balance in which two geometrically opposed high pressure chambers would cause a cancellation of radial load due to equal and opposed cross-section pressure areas and opposite vector direction which resulted in a zero net force radially on the shaft bearing. The design is referred to as the pressure balanced vane pump or motor. Typical efficiency of these devices is 70 to 85% under rated loading and speed. Still later improvements included changing the chamber shape of pressure balanced vane style devices and involved the use of several types of adjustable inner surfaces of the outer housing for guiding and radially adjusting the vanes as they rotate. One improvement is a continuous band which is flexible and subject to radial deformation so as to cause displacement control of the vanes. However, these flexible bands did not rotate.